The invention relates to a vehicle, in particular a hybrid vehicle.
A distinction is made with all-wheel-drive vehicles between a selectable all-wheel drive and a permanent all-wheel drive. With a selectable all-wheel drive, the vehicle front axle or the vehicle rear axle can be added if required. In contrast, the permanent all-wheel drive has a rigid drive train. With the rigid drive train, the drive torque generated in the drive unit is distributed substantially uniformly to the front axle and the rear axle via a mechanical interaxle differential, in particular a Torsen differential or a crown gear differential. In this case, individual vehicle wheels cannot be decoupled from the drive train. Rotation speed differences between the vehicle axles are hereby compensated by the interaxle differential.
Such vehicles may have a vehicle dynamic control system, which can be used to counteract swerving of the vehicle by intentionally braking individual vehicle wheels. A generic vehicle with such a vehicle dynamics control system is known from DE 198 43 221 A1, wherein the actual wheel rotation speeds detected by wheel sensors are used as input parameters. Depending on the detected actual wheel rotation speed, the lateral acceleration, the yaw rate and/or the steering angle, the vehicle dynamics control system performs automatic braking intervention on the vehicle brakes of the front and rear wheels. In addition, a brake booster is provided, with which the required actuating force on the brake pedal for attaining a braking effect desired by drivers can be reduced in a known manner. The brake booster according to DE 198 43 221 A1 is a so-called active vacuum brake booster which can be actuated both electrically via the control system and mechanically via a brake pedal.
Especially with vehicles having a partial or complete electrical drive, a recuperation torque, i.e. a drag torque or a negative drive torque, can be transferred to the vehicle wheels via the drive train in a recuperation mode, thereby decelerating the vehicle. This drag torque can cause substantial wheel-brake slip, especially on a road surface having a low friction coefficient (i.e. ice).
With a permanent all-wheel drive with an approximately uniform torque distribution, this brake slip is distributed approximately uniformly to all four wheels, so that both the front wheels and the rear wheels rotate uniformly more slowly due to the brake slip produced in the electric machine. A vehicle speed determined based on these measured actual wheel rotation speeds thus results in a vehicle reference speed that is too low, which misrepresents the actual vehicle speed, although it forms the basis for the vehicle dynamics control. The vehicle dynamic control system detects this condition as partial braking at a higher friction coefficient than is actually present. The actual vehicle speed is greater than that calculated by the vehicle dynamics control system from the wheel signals. This situation was referred to as a so-called “abseiled vehicle reference”.
The problem described above is specifically relevant for a permanent all-wheel drive. In contrast thereto, selectable all-wheel drives can disconnect the front axle or the rear axle from the drive train and calculate therefrom meaningful actual wheel rotation speeds that correlate with the actual vehicle speed. However, this type of decoupling from the drive train is not possible with a permanent all-wheel drive.